COHORT (Cooperative Huntington Observational Research Trial) is a coordinated research effort by Huntington Study Group research centers worldwide to prospectively collect data from consenting individuals who are affected by Huntington's disease (HD) and who are part of an HD family. The systematically accrued data from annual prospective assessments will relate clinical characteristics (phenotypes) between families with genetic and environmental factors. The knowledge from these relationships will better inform us about the onset and progression of HD, help identify potential interventions for HD, and aid in planning research studies of experimental treatments aimed at slowing or postponing the onset of HD. The consented collection of biological samples will further provide research material and correlative data for scientists to identify biomarkers that parallel the development and progression of HD. Identification of biomarkers will in turn contribute to our understanding of HD and enhance the efficiency and power of disease-modifying therapeutic trials.